Various situations exist in which it is desirable to rapidly determine the frequency of an incident RF signal, for example in order to determine what subsequent processing of the signal is appropriate.
In frequency-agile data links, for example, a data signal is transmitted using a sequence of changing carrier frequencies, in order to increase the rate of transmission of data and to reduce interference with other signals. Where the sequence to be used is not known, e.g. because it is determined dynamically in response to environmental conditions or the presence of signals on other wavelengths, optimal performance may necessitate detection of the carrier frequency in order to tune detector signal-processing circuits to that wavelength.
Similarly, frequency-hopping spread spectrum techniques are used in some applications for enhanced protection against interception. A signal that jumps rapidly between different frequencies is difficult to intercept, as it is often hard to distinguish from background noise. In such a system, it again can be necessary to detect the incident frequency and to tune the receiver circuit appropriately.
To take another example, the characteristics of active high impedance surfaces (HISs) offer advantages in many microwave circuit, antenna and defence applications. In such applications, it is desirable to detect rapidly the frequency of the incident radiation, for example, so that a varactor bias voltage can be determined and quickly applied, in order to tune the surface to an appropriate frequency (for example, in order to tune the surface so that there is a resonance at the incident frequency).
It has been found that commercially available RF frequency detectors typically have inadequate response times or are unable to handle the wide range of frequencies encountered in many applications (typical vector network analysers can estimate frequencies in a few milliseconds). Moreover, many detectors are bulky and not appropriate for situations (e.g. airborne applications) where component footprint, volume and mass are crucial.
Known high-speed frequency sensors typically sweep rapidly across a range of frequencies in which it is expected that the incident RF signal will be present. Typical sweep rates are dependent on the bandwidth of the swept filter (i.e. the resolution of the sensor): the narrower the bandwidth the slower the sweep frequency. A rule of thumb is that the sweep frequency is df/dt=B2/2.5 where B is the filter bandwidth. So for a 50 MHz filter bandwidth the sweep rate could be expected to be of the order of 1 GHz/μs.
The maximum speed of detection of an incident signal can be expected to be approximately equal to the time it takes to complete one sweep of the range of frequencies of interest, as, if the signal arrives at a frequency just after the detector has swept past that frequency, the signal will not be detected until substantially a whole sweep has been completed, and the relevant frequency is reached again. Such a system, using a 50 MHz bandwidth filter, would take 1 microsecond to perform a 1 GHz sweep, using a Gaussian filter profile, but there is a trade off between propagation delay and rate of fall off of the filter response at the band edge, and to achieve a rectangular filter profile would necessitate using a narrow filter and thus a longer sweep time.
Another prior-art technique is to digitise the incident signal data continuously in time, and then to perform a rolling-window Fast Fourier Transform (FFT) on the data. Nyquist theory predicts that to achieve a 1 GHz band with 50 MHz resolution, the data sample rate would need to be at least 2 GHz and the rolling window would need to be at least 20 ns wide. Although in principle this technique offers a good theoretical performance, several significant problems must be faced in practice. First, the computing overhead will significantly increase the propagation delay. Second, 2 GHz analogue-to-digital converters (ADCs) are not readily available. Third, the number of digital bits available with current 2 GHz ADCs is not sufficient to achieve a high dynamic range, with at best 30 dB achieved using this technique and currently available hardware.
Another well-known approach is the ‘frequency discriminator’ technique, in which a signal is passed through two paths of different lengths, and the phases of the two output signals are compared. The path length difference can be tailored to give a meaningful one-to-one relationship between phase and frequency over the desired operating range. In a typical prior-art implementation, an IQ mixer is employed to give output voltages indicative of the sine and cosine of the phase difference between the two paths. Twin ADCs are used to digitise those signals (the ADCs can run at a frequency equal to the desired bandwidth, i.e. 50 MHz, and electronics operating at that frequency is much more readily available than for the 2 GHz example given above). A processor converts that data into a signal which represents the input RF frequency.
There are several disadvantages with this approach. First, the approach gets more difficult as the total operation frequency band increases, as the path length difference must increase. Second, temperature effects change the delay length and have an impact on the accuracy of the conversion. Third, the approach only functions for one incident RF frequency; signals comprising more than one frequency can result in incorrect readings, as the discriminator can generate an output that is the mean of the two frequencies.
In some applications the performance of prior-art detectors is inadequate, for example where very short RF pulses are to be detected (of the order of a few tens or hundreds of microseconds), the operational frequency is large, a high dynamic range is required, and robustness to environmental changes (e.g. temperature changes) is desirable. It may be particularly desirable to detect a pulse before a substantial part of it has been incident on the detector, for example within a few hundred nanoseconds of the pulse arriving. In some applications, it is desirable to detect the pulse's carrier frequency as or very shortly after the leading edge of the pulse is incident on the detector. For such leading-edge detection, response times of a few tens of nanoseconds are desirable. We are not aware of any frequency sensors in the known art that can detect at high speeds the frequency of incident RF signals.
There is a need for a low-cost, rapid-response frequency detector capable of handling a wide range of RF frequencies.